An accelerator of this type, or, respectively, a triple pressure control valve equipped with such an accelerator, is known from German Published Application No. 26 51 377. According to that disclosure document, the pick-up accelerator is equipped with the intake and the shut-off valves only, the discharge valve being omitted. According to German Pat. No. 967,165, however, it is also known to provide a pick-up accelerator integrated into a triple pressure control valve with a discharge valve in addition to the intake valve.
According to German Published Application No. 26 51 377, the accelerator is structurally combined with the pick-up accelerator, in which the piston element which can be affected by the air main pressure against the reference chamber pressure, acts upon the shut-off valve, so that the shut-off valve simultaneously represents the valve arrangement coupled with the piston element. This prior art accelerator has the disadvantage that the discharge valve or, respectively, the valve system is coupled not only to the piston element but also to a small piston which is influenced by a brake pressure into the closing direction. The valve system therefore is precharged into the closing direction dependent on the braking level, so that differential pressures whose level is dependent on the respective prevailing braking level must act upon the piston element in order to be able to re-open the valve system during a braking operation. The prior art accelerator in its function hence manifests an undesirable dependency on the prevailing braking level.
The arrangement according to the aforementioned German Pat. No. 967,165 has only a pick-up accelerator but no piston element coupled with a valve system controlling an air outlet from the air main to atmosphere, said piston element being under the influence of pressure from the air main, acting against a reference pressure.
For an accelerator to be arranged separately from a control valve and, respectively, a triple pressure control valve having a pick-up accelerator, it is known to provide a piston element charged,on the one hand, by the pressure in one of the line chambers connected with the air main and, on the other hand, charged by the pressure in a reference pressure chamber, and being coupled with a valve system comprising an intake and discharge valve for the reference pressure chambers and with a discharge valve for the air main. The arrangement is so designed that the accelerator, when responding, first closes the intake valve to the reference pressure chamber by means of a supply of pressurized air, whereupon it opens the discharge valve from the reference pressure chamber by way of a choke connection to atmosphere, and only then opens the discharge valve for the air main. An accelerator of this type is known from German Pat. No. 969,085. As previously mentioned, these accelerators constitute additional components of the control valve and therefore demand additional investments in components and circuitry; these accelerators present a problem by the very difficult monitoring of the pressure in the reference pressure chamber, inasmuch as the said reference pressure chamber must, on the one hand, be protected from overcharges caused by filling strokes fed into the air main or the pressure source, respectively, while, during braking operations they must follow precisely the pressure decay in the air main, and pressure fluctuations in the pressure source influenced by factors thus may have an interfering effect. Accelerators of this kind, therefore, were used until now essentially only as so-called rapid acting brake accelerators responding only to particularly rapid pressure decays in the pressure main, such as would occur during emergency braking to assure proper functioning of such emergency braking.
Other so-called service brake accelerators are known which respond at the start and during the step-by-step increase of service braking operations and which, for the duration of the pressure decay within the air main, draw off pressurized air from the latter. These service brake accelerators often have an oscillating mode of operation, which means that they draw pulsating air from the air main. All of these service brake accelerators have a piston element which is charged by the pressure in a line chamber connected to the air main, against the pressure within a reference pressure chamber, the said reference pressure chamber in each case being connected with the air main by way of a nozzle. All of these accelerators, which are designed as components to be arranged separately from the control valve, have the basic disadvantage that the reference pressure chambers have no sure protection against overcharges such as may occur, for instance, during filling strokes, so that an undesirable response is possible, particularly during the decay of filling strokes. While it is possible to provide protective devices against filling stroke effects known from other arrangements, these devices would present an unbearable economic burden for the construction of accelerators of this type. An additional disadvantage of the aforementioned service brake accelerators can be seen in the nozzle connection of the reference pressure chamber with the air main: during the brake application, compressed air may flow from the reference pressure chamber into the air main, hindering the pressure decay in the air main, while during the brake release operations the transfer chambers may suck from the air main compressed air urgently needed at other locations, contributing in a manner undesirable to their recharging. Service brake accelerators of the aforedescribed kind are known, e.g., from German Published Application No. 29 46 657 and U.S. Pats. Nos. 4,073,544 and 4,145,090.
It is also known, from U.S. Pat. No. 4,206,949, to couple a service brake accelerator with pulsating operation with a control valve regarding its reference pressure chamber, in such a way that the reference pressure chamber of the accelerator is filled from a reference pressure chamber already provided within the control valve, i.e., that it is connected with this already available reference pressure chamber. The known accelerators of this type, however, during their operation influence the pressure within the reference pressure chamber of the control valve, and may thereby influence the operation of the control valve in an undesirable manner. As a minimum, coordination of the control valve and the service brake accelerator is necessary. Consequently, the aforementioned service brake accelerators cannot be simply combined with previously available control valves, and thus are unsuited for a retrofitting into existing pneumatic brake systems.